Shulou(Shulou.com)11/24 Report--

After the Mid-Autumn Festival

The sycamore leaves in Beijing are a lot of yellow.

There's a voice out of nowhere.

In the brain, in the stomach, in every cell:

"it's time to put on autumn fat!"

So steamed lamb steamed bear paw steamed deer tail,

Roast chicks, roast ducks, roast geese attacked me one after another.

And was successfully annihilated by me.

Every time I get on the scale, the editor is shocked by the jump of the weight meter.

When can the balance of the wallet be changed in this way?

Although weight is expressed in terms of mass

But the weight meter really suffers from the force!

Is it possible to--

It's not that I'm getting fat.

Is there something wrong with the gravitational constant?

There is no doubt.

The gravitational constant G is the most accurate physical constant that scientists have measured.

From Newton to today

The number of G often jumps horizontally.

Let's take a look today.

The story of the gravitational constant for three hundred years.

Gravity! Gravity! In 1687, Sir Newton wrote two paragraphs in the Mathematical principles of Natural philosophy:

The gravity of an object somewhere pointing to any planet is inversely proportional to the square of the distance from the place to the center of the planet.

All objects are attracted to each planet; the gravity of an object for any planet is proportional to the amount of matter contained in each object at an equal distance from the center of the planet.

With a big wave of his hand, Sir Alex wrote down the most important physics of gravity, but left a proportional coefficient G in the formula. This is the gravitational constant that has plagued scientists for more than 300 years.

For three hundred years, scientists have not been able to determine the value of gravitational constants accurately enough. Today, when the accuracy of most basic physical constants can be billions or even 1/10000000000, the accuracy of gravitational constants is still 1/100000.

Gravity is so weak that the vast universe is its stage; gravity is so hard to shield that you need a room for a laboratory-but the mass of cement and bricks interferes with the measurement. Even today, there are still a large number of the best physicists working tirelessly to improve the measurement of gravitational constants.

Three hundred years after the passage of time from Newton, generation after generation of scientists fell asleep all night and finally found several methods that can accurately measure the gravitational constant: the torsion balance compensation method, the torsion scale period method, the atomic beam or laser interference method and the most recent resonance method. They each have their own advantages, and together they calculate this annoying and fascinating physical constant.

Short stories of high school physics

More than a hundred years after Newton wrote the law of universal gravitation, no one has ever measured the specific value of the gravitational constant more accurately. It was not until 1798 when Cavendish of the University of Cambridge in England improved the precision torsion scale and completed the famous torsion scale experiment that it was written into the textbook. This experimental device, which was rated as one of the ten most beautiful experiments in the history of physics, is very exquisite and has become the blueprint for many precision measuring devices of later generations.

The torsion scale is a device suitable for measuring weak interaction forces. Its core component is a wooden pole hanging from a suspended wire, and different testing devices can be placed at both ends of the pole depending on the specific content of the experiment.

In the experiment of measuring the gravitational constant, a pair of equal mass metal balls with uniform density are placed at both ends, while in the Coulomb torsion scale experiment of measuring electrostatic strength, the testing device is a pair of small objects with the same amount of electric charge. Put objects near the testing device that can significantly change the force field, such as the one-ton shot put here, or the metal with a lot of electricity in the Coulomb experiment.

Schematic diagram of ▲ Cavendish torsion balance device

The testing device and force source at both ends of the pole are always arranged symmetrically, so the forces at the two ends are in the opposite direction, forming a force couple to make the torsion balance rotate and the wire twist. When the torsion recovery moment of the suspension wire is balanced with the applied couple moment, the torque can be measured by the torsion angle of the suspension wire, and then the force can be calculated. Since only the gravitational constant is left in the universal gravitation formula at this time, the calculation of G becomes a primary school math exercise.

Primary school mathematics is simple, but how to get reliable data that can be substituted into the formula is the difficulty. How to avoid the wind interference experiment caused by people walking around the room? How to accurately read the torsion angle of the suspended wire?

In order to shield the torsion scale from interference as much as possible, the torsion scale is placed in a wooden box, and people adjust the torsion scale outside the room through handles and knobs, so as to avoid the impact of human quality and air flow during operation.

In order to measure the twist angle of the suspended wire more accurately, Cavendish used light amplification. The collimated light beam emitted by the light source shines on the plane mirror fastened to the torsion scale and then reflected on the wall of the room. With the rotation of the torsion scale, the light spot on the wall also moves, thus transforming the tiny torsion angle of the torsion scale into a relatively long distance. When people read this distance outdoors by observing the mirror tube, another key data in the G measurement experiment can be obtained.

The freshman of torsion balance

Since the 20th century, worried about the inconvenience of the static measurement of the classical torsion scale, scientists began to use the torsion balance period method as one of the most commonly used methods to accurately measure the gravitational constant. The torsion balance period method is a dynamic measurement method, which mainly measures the changes of several physical quantities with time, so the interference that does not change with time in the static method can be removed.

▲ Tom: gravity doesn't exist as long as I don't bow my head

An ordinary free-hanging torsion scale will twist in the form of a damped oscillator, and the torsion angle is satisfied, in which the moment of inertia of the torsion scale (the position is equal to the mass in a straight line motion), the damping coefficient, which covers all the effects that attenuate the motion; it is the torsional elasticity coefficient of the torsion wire in the torsion balance. The intrinsic rotation frequency of the torsion scale is satisfied.

In the process of periodic measurement, two heavier objects are placed near the torsion scale as the "attracting mass". First, the two lines that attract the mass are parallel to the balance position of the torsion scale. at this time, the attraction will increase the total recovery torque of the torsion scale and speed up the rotation of the system, and the vibration frequency is the gravitational coupling constant determined by the mass distribution.

Next, the position of the attracting mass is adjusted so that the two lines are perpendicular to the balance position of the torsion balance, and the gravity of the attracting mass provides a negative recovery torque for the torsion balance system, so the rotation of the system becomes slower, and the rotation frequency is. In this way, the gravitational constant can be determined by subtracting the two equations. Among them, it is the square variance of the swing frequency of the torsion scale under the two configurations, and the sum is the difference between the sum of the two configurations.

Measuring G schematic diagram by cycle method of ▲ torsion balance

In the torsion balance cycle method, what we really need to measure dynamically is frequency-- that is, time. As it happens, time is a physical quantity that can be measured accurately. Instead of using light reflected from a plane mirror to measure the rotation angle, measuring time with a pulsed laser is as simple as a cat eating fish.

As for other parameters, such as moment of inertia, stiffness coefficient and even gravitational coupling constant, they are geometric parameters that do not involve the motion of the torsion scale-in short, they can be taken down and measured on the table. These relatively simple measurement requirements make the periodic method G measurement experiment can achieve a relatively high accuracy. However, the price of simple measurement is that the experiment lasts for a long time, so how to keep the environment stable without introducing new interference has become a problem that scientists need to solve.

Schematic diagram of the device used in the ▲ HUST-99 experiment

In order to obtain a stable external environment, scientists choose to carry out experiments in caves, with the help of thick mountains to shield from external vibrations. In 2006, Chinese scientists used the torsion balance periodic method (in Wuhan caves) to measure the value of gravity constant with a relative error of 130 parts per million, which became one of the eight recommended values of gravity constant adopted by the International Commission of Science and Technology, and appeared in the textbook of high school physics.

Am I getting fat again?

Recently, a group of Swiss scientists gave a measurement of the gravitational constant using a new dynamic resonance method. The gravitational field of one vibrating beam will cause the weak vibration of another beam, and the value of the gravitational constant can be calculated by measuring the periods of two vibrations by laser. They made 18 measurements and got an average of 1.66% uncertainty. In other words, the value of the gravity constant measured this time is 2.2% higher than the existing internationally recommended value. if that is the case, the editor will have to gain three jin before the holiday season. ).

Source of ▲ original picture, source of translation

Although the uncertainty of the new method is still larger than that of traditional methods such as torsion scales, even considering the uncertainty of 1.66%, the value measured by the new method is still a little larger than the current standard value. This may be because the new method is more accurate, but it is also possible that there are some systematic errors that have not been excluded-after all, more than 200 years from Cavendish to now, the results measured by different torsion scales around the world are still very close.

Of course, it may simply be that the gravitational constants in Swiss caves are different from those in other parts of the world-scientists have previously claimed that gravitational constants vary with position and even measurement scale. If so, whether this proportional coefficient in the formula of universal gravity can still be called a "constant" may be another thing worthy of scientists' brains and drooling.

In any case, today, it is classic enough, stable enough and enough for daily use. The constant pursuit of the precise measurement of gravity constant is more about the pursuit of the truth of physical theory and the climbing of the peak of precision measurement technology-- the development of theory and technology is certainly not a waste and will certainly benefit people's lives. useless babies will eventually grow into giants of the next era.

▲ Faraday thought it was great.

It doesn't matter how many jin people gain because of the change of gravitational constant. It's important to have enough to eat. The crabs are fat and the bass are delicious. May our foodie readers eat and drink well and gain weight.

Note: a Faraday was mentioned in the high school physics textbook: when Faraday first invented the generator, a British lady asked him what was the use of electricity. Faraday replied, "Ma'am, what is the use of a newborn baby?"

reference

[1] by Newton, Wang Kedi. Scientific Literacy Library Science Classics Series: mathematical principles of Natural philosophy [M]. Peking University Press, 2013.

[2] what are the significance and difficulties of measuring the "universal gravitation constant" with high precision? -the answer of the crab who missed the net.-Zhihu

Https://www.zhihu.com/question/375778009/answer/1112711841

Tu Liangcheng, Li Qing, Shao Chenggang, Hu Zhongkun, Luo Jun. Accurate measurement of gravitational constant G [J]. Chinese Science: physics, Mechanics and Astronomy, 2011 Journal 41 (06): 691-705.

Zhao Jinquan, Zhang Shuo, he Yanlan. Measurement of gravitational constant by laser interferometry [J]. College Physics, 2007 (12): 53-56-59.

[5] Brack, T., Zybach, B., Balabdaoui, F. Et al. Dynamic measurement of gravitational coupling between resonating beams in the hertz regime. Nat. Phys. 18,952-957 (2022).

[6] Nie Fuyuan The gravitational constant was determined for 200 years [J]. Physics Bulletin, 1998 (07): 37-38-44.

He Wenyu, Wang Xiaojun. Teaching discussion on the inquiry process of deep reduction physicists-- taking Cavendish torsion balance experiment as an example [J]. Physics teacher, 2019. 40 (11): 20-23.

Tu Liangcheng, Luo Jun. New progress in experimental and theoretical research of gravity [J]. Advances in Natural Science, 2005 (08): 897906.

[9] https://mp.weixin.qq.com/s/WcykLHS1kNSSRG5jfLk83A

Memes come from the Internet.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Tibetan idiot

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.